This invention relates to a unique method of minimizing the detrimental effect of compressor flooded starts by running the compressor in reverse for a variable period of time.
A scroll compressor is one type compressor widely utilized in refrigerant compressor operation. One known type of scroll compressor includes compression elements and an electric motor housed within a sealed compressor shell. A quantity of lubricant is also received in the compressor shell. In such compressors, the refrigerant passes over the motor on its way to the inlet of the compression elements, cooling the motor.
In a scroll compressor a pair of scroll members have wraps which interfit with each other to define compression chambers. When rotated in a forward direction, a normal compression process occurs in which refrigerant is trapped between the wraps and compressed towards a discharge port.
At startup, oil located in a compressor sump, may contain a quantity of liquid refrigerant. At startup the sump and motor are cool, and preheating does not occur. The presence of the non-preheated oil/refrigerant mixture in the oil sump has undesirable effects.
The problem is particularly acute in refrigerant systems for intermodal transport, here refrigeration takes place in large containers used to transport fruit or other food products over long distances. Inter-modal refrigerant containers may be initially shipped on a boat, transferred to a train, and then transferred to trucks. Refrigeration must be maintained throughout the entire trip. The container may then be returned to a remote location for storage. Thus, during its life cycle a container refrigeration system may often be shut down for long periods of time.
The problem can become especially severe during cold starts. When the refrigeration system is shut down, under certain ambient conditions, large portions of liquid refrigerant contained in the system can migrate to the compressor sump. Thus oil located in the compressor sump can be diluted with liquid refrigerant. This is undesirable because on startup it can result in liquid refrigerant passing through the scroll elements. This can result in bearing and scroll element damage. Also oil dilution by the refrigerant may lead to lower lubricant viscosity, which is also detrimental to bearing life.
It is known for a scroll compressor to be operated in reverse at startup. See U.S. Pat. No. 6,648,604. When operated in reverse, the refrigerant is not compressed and is not moved through the compression elements. Thus, motor heat is not removed by refrigerant vapor. The motor is immersed in the oil/refrigerant mixture and thus quickly heats this mixture. Refrigerant trapped in the sump is then boiled off and oil temperature is increased.
Additional boiling off of refrigerant takes place as the oil/refrigerant mixture has been agitated by the electric motor rotor and rotor counterweight, rotating in the oil/refrigerant mixture. After a short period of time reverse rotation is stopped, and the motor is rotated in a forward direction and normal compression begins. However, the oil is now preheated and there is less liquid refrigerant in the oil.
The technique above has been used with scroll compressors, and can also be applied to other compressor types such as for example screw compressors.
The severity of the flooded start is greatly minimized by increasing the time of the reverse run. However, increasing this time can cause undesirable overheating of the compressor components, as almost all the heat generated by the motor in reverse run is dissipated within the compressor shell. This problem is especially acute at high ambient condition, when the overheating occurs quickly (15 to 25 seconds). In this case, the compressor might trip an internal line break (which is undesirable, since the compressor will not be able to start up on demand) and/or compressor reliability is sacrificed as generated heat can damage motor laminations and score scroll elements. A similar situation also may occur if on start up the compressor motor was voltage falls below the optimal voltage. In such a case, undesirable overheating may also occur.
Therefore a need exists to further optimize the reverse run operation at conditions when overheating is likely when the compressor runs in reverse.